Injections of IgD-containing ascites or purified IgD into BALB/c mice cause enhancement of antibody production to trinitrophenylated (TNP) conjugates. Splenic T cells from IgD pretreated mice transfer this enhanced ability to produce antibody to normal recipients. Experiments are proposed to characterize the cells response for this effect and to analyze the regulatory events involved in this phenomenon at the molecular level. The suggested approach is based on the working hypothesis that T cells with Fc receptors for IgD down-regulate IgD transcription and simultaneously stimulate IgM secretion and/or secretion of other Ig isotypes.